Digital video compression methods work by exploiting data redundancy in a video sequence (i.e., a sequence of digitized pictures). There are two types of redundancies exploited in a video sequence, namely, spatial and temporal, as is the case in existing video coding standards. A description of these standards can be found in the following publications, which are hereby incorporated herein by reference: (1) ISO/IEC International Standard IS 11172-2, “Information technology—Coding of moving pictures and associated audio for digital storage media at up to about 1.5 Mbits/s—Part 2: video,” 1993; (2) ITU-T Recommendation H-262 (1996): “Generic coding of moving pictures and associated audio information: Video,” (ISO/IEC 13818-2); (3) ITU-T Recommendation H.261 (1993): “Video codec for audiovisual services at p×64 kbits/s”; (4) Draft ITU-T Recommendation H.263 (1995): “Video codec for low bitrate communications.”
Compressed video streams typically include independent and dependent frames. An independent frame is a frame that is coded using information only from itself whereas a dependent frame is a frame that is coded using information from one or more other frames. An example of an independent frame is an Intra frame (I-frame). I-frames are compressed by exploiting the internal spatial redundancy of each macroblock independently of reference pictures. The first picture of a picture sequence is typically, but not necessarily, an I-frame. Examples of dependent frames are Predictive frames (P-frames) and bi-directional frames (B-frames). P-frames are pictures in which macroblocks can be compressed by predicting their value from a past reference picture. A past reference picture is a picture, either an I or another P-frame that is to be displayed prior to the current picture. B-frames are pictures in which macroblocks can be compressed by predicting their value from a past reference picture and a future reference picture. A future reference picture is a picture, either an I or another P-frame that is to be displayed after to the current picture.
A compressed video stream may be used to provide trick modes such as, for example, fast-forward playback and reverse playback. Providing trick modes for a compressed video stream has traditionally required the presence of I-frames in the video stream. For example, U.S. Pat. No. 6,057,832 (Lev et al.) discloses one method whereby MPEG-2 stream containing I-frames may be used to provide trick mode functionality. However, some MPEG-2 streams such as for example, HITS (Headend In The Sky) streams, do not contain I-frames or a typical grouping of pictures and are therefore incapable of providing multiple playback modes using traditional methods. Therefore, there exists a need for systems and methods for providing trick mode functionality in connection with compressed video streams that do not include a certain type of frames.